This invention relates to magnetic recording disks for use in perpendicular magnetic recording systems, such as rigid disk drives.
Perpendicular magnetic recording, wherein the recorded bits are stored in a perpendicular orientation in the media, is considered to be a promising path toward ultra-high recording densities in magnetic recording rigid disk drives. While perpendicular magnetic recording rigid disk drives have not yet been commercially realized, several types of perpendicular magnetic recording media have been proposed.
The most widely reported type of perpendicular media is a cobalt-chromium (CoCr) alloy xe2x80x9cgranularxe2x80x9d layer grown on a special growth-enhancing sublayer that induces the crystalline C-axis to be perpendicular to the plane of the layer, so that the layer has strong perpendicular magnetic anisotropy. This type of media is called xe2x80x9cgranularxe2x80x9d because the individual grains are only weakly exchange coupled laterally.
To improve the signal-to-noise ratio (SNR) and the resolution in perpendicular magnetic recording media, including the CoCr granular type of media, the thickness of the medium is decreased. However, a thin perpendicular medium has a large demagnetizing field at the top surface. Thus while the CoCr granular type of perpendicular media may have good SNR, it is vulnerable to demagnetization and thermal decay at low spatial frequencies, because strong demagnetizing fields exists at the center of large bit cells. The centers of such bit cells have been reported to undergo severe demagnetization and thermal instability in CoCr granular perpendicular media, as reported by Hirayama et al., xe2x80x9cMagnetization Decay in CoCr-alloy Perpendicular Magnetic Recording Mediaxe2x80x9d, Journal of the Magnetics Society of Japan, Vol. 21, Supplement, No. S2, pp. 297-300 (1997).
Another type of perpendicular media is a xe2x80x9ccontinuousxe2x80x9d multilayer or superlattice of alternating films of Co with films of platinum (Pt) or palladium (Pd), which have been shown to have strong perpendicular magnetic anisotropy. Carcia et al., xe2x80x9cPerpendicular Magnetic Anisotropy in Pd/Co Thin Film Layered Structuresxe2x80x9d, Appl. Phys. Lett., Vol. 47, pp. 178-180 (1985). They are called xe2x80x9ccontinuousxe2x80x9d because the grains in the films are strongly exchange-coupled laterally. These multilayers and have been shown to possess very high thermal stability, even at large bit separation, as reported by Honda et al., xe2x80x9cTime Dependence of Magnetic Properties in Perpendicular Recording Mediaxe2x80x9d, IEICE TRANS. ELECTRON., Vol. E80-C, No. 9, pp. 1180-1185, September 1997. However, the medium noise for this type of continuous exchange-coupled media is considerably higher than for the CoCr granular media, resulting in low SNR values. Suzuki et al., xe2x80x9cRead-Write Characteristics of Co/Pt Multilayer Perpendicular Magnetic Recording Mediaxe2x80x9d, Journal of the Magnetics Society of Japan, Vol. 18, Supplement, No. S1, pp. 451-454 (1994). The large medium noise is due to the strong lateral exchange-coupling between the Co grains in the multilayer, which results in large magnetic domain sizes. Wu et al., xe2x80x9cNoise Origin in Co/Pd multilayer Media for Perpendicular Magnetic Recordingxe2x80x9d, IEEE TRANSACTIONS ON MAGNETICS, Vol. 33, No. 5, pp. 3094-3096, September 1997.
What is needed is a perpendicular magnetic recording medium that has high SNR and good thermal stability.
The invention is a composite perpendicular magnetic recording disk that has two distinct magnetic layers formed on the disk substrate, each with strong perpendicular magnetic anisotropy, that are strongly exchange coupled perpendicularly to each other across their interface. In the preferred embodiment the first layer is a CoCr granular layer formed on a growth-enhancing sublayer, such as titanium, so as to have strong perpendicular magnetic anisotropy. The second layer is a continuous layer, preferably a continuous multilayer of Co/Pt or Co/Pd formed on top of the granular layer. An interface layer, such as a layer of platinum or palladium, depending on whether the top multilayer is Co/Pt or Co/Pd, respectively, may be located between the two layers to enhance the growth of the continuous multilayer and to moderate the strength of the exchange coupling between the granular layer and the continuous multilayer. If the composite perpendicular magnetic recording disk is to be used in a rigid disk drive that uses a pole type read/write head, a soft magnetically permeable underlayer is located on the disk substrate beneath the granular layer. If the composite disk is to be used in a disk drive that uses a ring type head, where a soft underlayer may not be necessary, then the continuous multilayer may be the bottom layer located on the substrate and the granular layer may be the top layer located on the continuous multilayer. In both cases, there is strong perpendicular exchange coupling at the interface between the granular layer and the continuous layer. As an alternative to the Co/Pt or Co/Pd multilayer as the continuous layer, the continuous layer may be a single layer of chemically ordered CoPt, CoPd, FePt, FePd, CoPt3 or CoPd3. The composite medium has a higher SNR at high linear recording densities than either a CoCr granular layer alone or a Co/Pt continuous multilayer alone. as well as thermal stability approaching that of a Co/Pt continuous multilayer.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.